3 Answers
3

Stirges are definitely an under-rated creature for any edition of D&D. In old school D&D I'd hesitate to put a flock of them up against 1st level characters, they ability to drain blood could conceivably wipe out a low level party! However, they DO make a good nasty foe for a low level party in small numbers, especially if the party has been warned if they are about. Here are some examples how I use them (or could use them):

When I use stirges for low level parties I would first give some sort of warning that a flying menace is about (drained bodies, etc). Then I would hit the party with 1-2 of them to warn them that there may be a nest about, and let the fun ensue! In early D&D editions they only have 1+1 HD but attack as 4th level monsters, which could be tough....this plus their ability to latch on and drain blood should spell the end of most low level characters in a round or two unless they are clever or have a good strategy.

In larger numbers they could be quite more than a simple menace to even a high level party. If they swarm the party in low light conditions, and several of them fasten on, they could cause quite a few problems. Don't forget that characters with stirges attached cannot strike at them with larger weapons (at least that's what I rule) and have pull them off or use short hand held weapons or lit torches to dislodge them....not to mention the fun and games should a companion begin striking at stirges attached to one of his buddies! I wouldn't hesitate to have a large nest of say 20-30 of the critters take on a higher level party due to the havoc they could wreak. Of course, if warning is given there are plenty of spells that could finish off a flock fairly quickly, but if nothing else this wastes party resources.

To make them a full fledged menace I'd have the encounter in a dark, very tight space (say, a 5x5 cavern passageway), coming out of small holes in the ceiling and walls, where a spell like Fireball, Web or the like is truly impractical. Forcing a party to hack through a flock of these in a narrow passageway not only is tough but could alert other monsters or baddies ahead....they could be used by a tougher monster (say, a minotaur or vampire) as a warning system for their lair.

A Stirge in 4e is actually quite a fun creature. They deal ongoing damage while attached, instead of ability damage (which happily no-longer exists). The real challenge is escaping from their grab, as they get a tremendous AC boost while grabbing.

However, as their Fortitude and Reflex are equivalent to a moderate skill-check to escape grabs through athletics or acrobatics, an encounter with stirges will mostly force the party to use different tactics (party-unfriendly area attacks become functionally unusable) but no character is simply "eliminated" from the fight nor dropped horribly quickly.

The mechanics mean that stirges are a relevant threat in any number, as their grab-tactics demand high-priority, though they don't do tremendous amounts of damage. With high numbers, they become seriously vulnerable to controllers willing to "back scratch" with damaging powers, but do not pose a party-ending threat.

The Stirge swarm, on the other hand, is quite lethal as swarms tend to be, and should be used with extreme moderation.

It somewhat depends on version, as in some editions they are a lot more dangerous than others. Compare the 3.5e version (1d4 CON damage a round) with the Pathfinder version (1 CON damage a round).

In any event, CON damage is nothing to mess around with. It takes your hit points down quick, and can kill even a high level PC quickly. And though it takes a couple stirges to kill one person, they have a disproportionate effect in long term weakening of the PC - healing ability damage requires higher level spells and means bad Fort saves and hit points for days while it heals.

They're only a "kill outright" threat to low levelers unless they're in bulk, but as a random encounter they definitely weaken a party a lot more than something else.